Reciprocating piston internal combustion engines utilize valves to control the flow of intake air and exhaust to and from each cylinder or combustion chamber. The valves may be opened and closed in a variety of manners. In a common configuration, a stem extends from the valve. A spring biases the valve to a closed position and the free end of the stem is selectively depressed to move the valve to an open position. In one configuration, the stem is depressed with a rocker arm. The rocker arm is mounted for back and forth or rocking movement. When one end of the rocker arm is moved (such as directly by a camshaft or indirectly via a lifter/pushrod), the other end of the rocker arm selectively presses upon the stem and moves the valve.
Generally, a small gap is provided between the rocker arm and the stem. This gap is known as the valve clearance. Over time, the valve clearance will change. Among other things, wear of the stem and rocker arm may cause the valve clearance to increase. Changes in the valve clearance may result in accelerated wear and damage to engine components and change the valve opening timing. As such, it is desirable to maintain proper valve clearance.
A variety of rocker arm mounting configurations have been proposed, all of which have drawbacks. In one configuration, the rocker arm may be mounted to a threaded stud which extends out from a cylinder head of the engine. In this configuration, the rocker arm is generally free to float or move in every axis relative to the stud, as movement of the rocker arm is only indirectly constrained by devices that restrict the unwanted side-to-side movement of the lifter which actuates the rocker arm. Valve clearance may be adjusted by changing the position of the rocker arm along the threaded stud (moving the rocker arm up and down) and by securing the rocker arm in place using a threaded nut. This rocker arm mounting configuration has a number of drawbacks. First, it is difficult to position and secure the rocker arm in a desired position. Second, the rocker arm mounting allows for too much rocker arm movement, which is only acceptable for engines which operate at low speeds. Third, the trunnion is not rigidly fixed to the stud.
Another rocker arm mounting configuration is illustrated in FIGS. 1A and 1B. As illustrated, a rocker arm RA is mounted to a rocker shaft RS which is attached to a rigid base that is attached to a cylinder head of the engine. In this configuration, the rocker arm RA is only allowed to rotate in a single desired plane. This type of mounting thus works well in high engine speed applications.
However, valve clearance adjustment is complicated. As illustrated, the actuated end of the rocker arm RA includes a roller tip RT. The opposing actuating end of the rocker arm RA (which is configured to moved by a pushrod P which is actuated by a cam C via a lifter L) includes a jam nut N and adjusting screw AS. Movement of the adjusting screw AS relative to the rocker arm RA allows the valve clearance relative to a stem S of a valve V to be adjusted. One disadvantage to this configuration is that the adjusting screw AS is not only the adjustment mechanism, but comprises the point of engagement with the rocker arm RA. The adjusting screw AS is thus subject to extreme wear and tear at the same time as it is supposed to maintain an exacting position.
To overcome this problem, the adjusting screw is often removed to increase fatigue life, reduce weight and provide room for greater rocker arm ratios. In that case, however, clearance adjustment must be accomplished with yet another mechanism, such as a complex system of variable height hardened valve caps which are interchanged.
An improved rocker arm mounting configuration including valve clearance adjustment is desired.